Flat-panel display devices, for example plasma, liquid crystal and Organic Light Emitting Diode (OLED) displays have been known for some years and are widely used in electronic devices to display information and images. Such devices employ both active-matrix and passive-matrix control schemes and can employ a plurality of colored light-emitting elements to form a full-color, pixilated display. Each pixel comprises a plurality of colored sub-pixel light-emitting elements, for example red, green, and blue. It is also known to provide color displays with four colored sub-pixels in each pixel of a full-color display to reduce power usage, for example as taught in U.S. Pat. No. 6,919,681 by Cok et al. The light-emitting elements are typically arranged in two-dimensional arrays with a row and a column address for each light-emitting element and having a data value associated with each light-emitting element to emit light at a brightness corresponding to the associated data value. In some displays, the colored sub-pixels are formed in rows or columns of a common color; in other displays sub-pixels of the same color may be offset from each other in neighboring rows or columns.
In general, displays suffer from a variety of defects that limit their quality. In particular, displays may suffer from defective light-emitting elements that do not respond properly to control signals, for example, the defective light-emitting elements may be permanently turned on, permanently turned off, be brighter, and/or be dimmer than intended for a given control signal. These non-uniformities can be attributed to the light-emitting or light-controlling materials in the display or, for active-matrix displays, to variability in the thin-film transistors used to drive the light emitting elements. Moreover, applicants have determined through experiments that defective light-emitting elements vary in the accuracy of their response at different brightness levels so that a light-emitting element may have a more accurate response at some light levels than at others. In other words, a pixel may be defective at one light level but less defective or not defective at all at another light level. Furthermore, most displays are color displays having pixels with three or four colored light-emitting elements and defects may be found in one color light-emitting element of a display pixel but not in the other color light-emitting elements of the same pixel. Such defects reduce the quality, reduce the manufacturing yields, and increase the costs of flat-panel displays.
A variety of schemes have been proposed to correct for non-uniformities in displays. Many such schemes are addressed to improving the uniformity of the light-emitting elements, for example, copending, commonly assigned U.S. Ser. No. 10/869,009, filed Jun. 16, 2004, describes providing an OLED display having a plurality of light-emitting elements with a common power signal and local control signals; providing a digital input signal for displaying information on each light-emitting element, the signal having a first bit depth; transforming the digital input signal into a transformed digital signal having a second bit depth greater than the first bit depth, and correcting the transformed signal for one or more light-emitting elements of the display by applying a local correction factor to produce a corrected digital signal. However, such uniformity correction schemes may not sufficiently correct for display devices having defective light-emitting elements that are stuck on or stuck off, or that are not sufficiently responsive to control signals to perform the desired correction.
Copending, commonly assigned U.S. Ser. No. 11/040,066, filed Jan. 21, 2005, describes a further correction method for compensating output of defective light-emitting elements in a display. Referring to FIG. 5, e.g., in a specific embodiment discussed therein a display having pixels 38 with sub-pixels 30, 32, and 34 forming a color gamut (e.g. red, green, blue), a defective sub-pixel 20 may be compensated by driving the nearest sub-pixels of a common color (22, 24, 26, 28). However, such an approach often creates visible spatial artifacts because of the distance between the sub-pixels of a common color.
Some further correction methods for masking stuck light-emitting elements in a display are known. For example, WO/2005/052902 describes a method for reducing the visual impact of defects present in a matrix display comprising a plurality of pixels, said pixels comprising at least three sub-pixels, each sub-pixel intended for generating a sub-pixel color which cannot be obtained by a linear combination of the sub-pixel colors of the other sub-pixels of the pixel, the method comprising: providing a representation of a human vision system, characterising at least one defect sub-pixel present in the display, the defect sub-pixel intended for generating a first sub-pixel color, the defect sub-pixel being surrounded by a plurality of non-defective sub-pixels, deriving drive signals for at least some of the plurality of non-defective sub pixels in accordance with the representation of the human vision system and the characterising of the at least one defect sub-pixel, to thereby minimise an expected response of the human vision system to the defect sub-pixel, and driving at least some of the plurality of non-defective sub-pixels with the derived drive signals, wherein minimising the response of the human vision system to the defect sub-pixel comprises changing the light output value of at least one non-defective sub-pixel for generating another sub-pixel color, said another sub-pixel color differing from said first sub-pixel color. The invention also provides a corresponding system for reducing the visual impact of defects present in a matrix display, and a matrix display with reduced visual impact of defects present in the display. However, there is no teaching with respect to optimal correction for defective pixels in systems in which combinations of sub-pixels can generate a color found in an other sub-pixel, for example as taught in U.S. Pat. No. 6,919,681 referenced above.
WO2003100756 addresses issues found with displays having at least one redundant sub-pixel. This application describes a method for masking faulty sub-pixels in a display having a plurality of pixels formed of a number of sub-pixels, wherein at least one pixel in said display is faulty and comprises at least one sub-pixel having a defect. The method comprises obtaining a set of sub-pixel values for generating desired perceptive characteristics for said pixel and determining a modified set of sub-pixel values for generating modified perceptive characteristics for said pixel. This modified set of sub-pixel values is based on information regarding the sub-pixel defect so as to be implementable in the display, and has values chosen to reduce an error perceived by a user. The modified values are then implemented in the display. The display is preferably of the kind where each pixel comprises a set of primary sub-pixels each emitting a primary color and at least one additional, redundant sub-pixel for emitting an additional color, such as a RGBW display. The disclosure includes a reference that the optimization problem can also be extended to include the distance to surrounding sub-pixels, but no solutions are taught. Referring to FIG. 6, in a prior-art illustration, a display having pixels 35 with sub-pixels 30, 32, and 34 forming a color gamut (e.g. red, green, blue), with an in-gamut sub-pixel 36, a defective in-gamut sub-pixel 40 may be compensated by driving the color sub-pixels 50, 52, and 54 of the same pixel 58. However, in this design, the spatial extent of the corrected light-emitting elements is not optimized, and is thus more visible than may be necessary.
There is a need, therefore, for an improved display that compensates for defective light-emitting elements in a full-color display.